refactor: retire tui.BuiltinThemes in favor of theming.List

Make internal/theming.List the single source of truth for themes:

- Drop tui.BuiltinThemes, tui.ThemeByID, and the NvChadTheme field on
  tui.Theme. tui.Theme is now a thin {ID, Name} display value.
- Wizard populates its dropdown from theming.List() and uses
  DisplayName for the label.
- neovim.resolvedTheme uses theming.Get (with theming.List()[0] as a
  fallback) instead of a caller-supplied []tui.Theme slice.
- neovim.New signature loses the themes []tui.Theme parameter; update
  cmd/archduino/main.go and all neovim tests to match.

Break the resulting neovim→theming import cycle (theming previously
imported neovim for WriteChadrc) by inverting the dependency:
theming exposes a ChadrcWriter function hook, and neovim registers
its WriteChadrc into it via package init. The theming test stubs
ChadrcWriter directly to exercise the chadrc-rewrite branch without
pulling neovim in and re-creating the cycle.

Delete the now-obsolete TestBuiltinThemesCarryNvChadName / TestThemeByID*
tests in internal/tui/state_test.go — their coverage lives in the
theming package's own tests.

internal/theming/theming.go

@@ -13,11 +13,17 @@ import (
 	"strings"
 
 	"github.com/MDietermann/ArduinoArchSetup/internal/config"
-	"github.com/MDietermann/ArduinoArchSetup/internal/modules/neovim"
 	"github.com/MDietermann/ArduinoArchSetup/internal/platform"
 	"gopkg.in/yaml.v3"
 )
 
+// ChadrcWriter is the hook used by Apply to rewrite NvChad's chadrc.lua
+// after a theme change. The neovim module registers its WriteChadrc here
+// via an init() function to avoid a theming→neovim import cycle (neovim
+// now imports theming to look up the NvChad theme name). If nil, Apply
+// skips the chadrc update silently.
+var ChadrcWriter func(nvChadThemeName string) error
+
 //go:embed assets
 var assetsFS embed.FS
 
@@ -146,8 +152,10 @@ func Apply(cfg config.Config, themeID string) error {
 		return fmt.Errorf("update overlay: %w", err)
 	}
 
-	if err := neovim.WriteChadrc(th.NvChadTheme); err != nil {
-		return fmt.Errorf("update chadrc: %w", err)
+	if ChadrcWriter != nil {
+		if err := ChadrcWriter(th.NvChadTheme); err != nil {
+			return fmt.Errorf("update chadrc: %w", err)
+		}
 	}
 
 	// Clear NvChad's base46 theme cache so the new theme activates on next